1. Field of the Invention
The present invention relates to a motor-driven compressor incorporated in an automotive air conditioning system.
2. Description of the Related Art
An air conditioning system for a motor vehicle includes a compressor for compressing a refrigerant and discharging the resultant high-pressure refrigerant. The high-pressure refrigerant is used to cool the interior of the vehicle compartment of the motor vehicle. More specifically, the high-pressure refrigerant is liquefied by a condenser and then evaporated in an evaporator to cool the air which is introduced into the vehicle compartment.
As such compressor, a motor-driven compressor directly coupled with an electric motor is known. The motor-driven compressor includes an electric motor and a compression unit driven by the electric motor, and the compression unit has a swash plate-type compression mechanism, for example, built therein.
In the height of summer, the air conditioning system is required to provide high cooling power and the electric motor of the motor-driven compressor is operated at high speed for a long time. In such cases, the quantity of heat generated by the electric motor greatly increases, so that the ambient atmosphere of the electric motor is heated to an extremely high temperature. As a result, the rotation efficiency of the electric motor lowers, making the air conditioning system unable to provide the required cooling power.
To eliminate the drawback, a motor-driven compressor disclosed in Japanese Patent Application Publication No. 2001-193639 is provided with a built-in cooling circuit, and the cooling circuit allows part of the suction refrigerant, which is being returned from the evaporator to a suction chamber of the compression unit, to flow into the motor-driven compressor. More specifically, the part of the suction refrigerant first flows into an armature chamber of the electric motor, then passes through a swash plate chamber, that is, a crank chamber of the compression unit, and returns to the suction chamber. This suction refrigerant flow serves to cool the electric motor as well as the compression unit, thereby preventing overheating of the electric motor and the compression unit.
However, after the refrigerant is used for cooling, the temperature thereof is high since the refrigerant has absorbed the heat of the electric motor and compression unit. If such high-temperature refrigerant is returned to the suction chamber, the temperature of the suction refrigerant to be compressed by the compression unit undesirably increases, and the increase in the temperature of the suction refrigerant significantly lowers the refrigerant compression efficiency of the compression unit, that is, the cooling power of the air conditioning system.
Also, when passing through the crank chamber of the compression unit for cooling same, the refrigerant carries off lubricating oil in the compression unit, with the result that the lubricating oil in the compression unit runs short, hindering smooth operation of the compression unit.